This invention in general relates to optical radiation sources and in particular to improvements in lasers suitable for use in optical communication systems.
Communication by means of optical electro-magnetic radiation transmitted along optical fibers is now a well-established practice made possible by the development over the past 25 years of reliable, coherent sources, low-loss optical transmission fibers, and suitable detectors. Motivating this rapid progress was the proposal of the optical maser by A. L. Schawlow and C. H. Townes in 1958 and the subsequent announcement by T. H. Maiman in 1960 of its actual reduction to practice with laser action in ruby. With the achievement of the coherent, or quasi coherent, source came the recognition that the efficient transmission of radiation from laser sources along glass optical fibers of suitably low attenuation could provide communication systems which offered significant advantages over metallic cables, the most important of which was dramatically higher information carrying capacities. In addition, compactness and substantially reduced weight along with lower manufacturing and installation costs were also incentives adding impetus to this progress to the point where now systems operating near the theoretical shot noise limit for signal power at the detector and information capacity in a given wavelength interval are under consideration for installation in the field.
The higher information capacity of optical systems exists because a communication channel requires the same bandwidth regardless of the region of the spectrum in which it is located, and the higher-frequency regions, the optical regions, have far more room for channels and consequently have a much greater potential capacity than the lower competitive frequencies. To fully exploit this capacity in an economic way, however, places certain requirements on optical sources for use in such systems.
The basic characteristics for a light source for use in communications applications, and those which govern achievable system performance, include the source spectral emission such as wavelength, line width, and wavelength stability, power output, physical size, power efficiency, life, coherence, cost, and modulatability.
Source spectral emission must complement the optical fiber attenuation and dispersion properties if efficient use of source power is to be made. Attenuation characteristics of optical fiber waveguides varies as a function of wavelength, generally decreasing with increasing wavelength. Regions from 0.8 to 1.6 micrometers have attractive low-loss transmission. At the shorter wavelength end, loss is sufficiently low for many applications, but where maximum distance is to be covered between repeaters, the longer spectral region from 1.2 to 1.6 micrometers is more suitable.
Source spectral width is also an important consideration since the refractive index of fiber material also varies with wavelength. This latter property, known as material dispersion, causes pulse spreading which reduces the data rate capacity of the fiber, and the pulse spreading is more severe with spectrally wider sources than with narrower. Consequently, it is important that the spectral width of the source be as narrow as possible to be consistant with high data rate transmission. Inasmuch as the spectral emission characteristics of the source vary with temperature, it is important to keep in mind how these changes occur and to provide appropriate temperature control where necessary for the application in mind.
In wavelength division multiplexing applications, it is important that the width of the spectral emission of the source be as narrow as possible to achieve a high density of multiplexing. For wavelength multiplexing, the line width and its shape directy influence adjacent channel cross-talk levels. A set of sources with a narrow emission spectrum of a few tenths of a nanometer, forming a set of non-overlapping spectral sources, can be used effectively to achieve wavelength multiplexing within a spectral range. If these sources have wider emission spectrum than the overlapping criterion would allow, selective filters can be used.
In general then, the source spectral output should be in a region where fiber attenuation is low and should be of narrow bandwidth to minimize dispersion effects and maximize both channel density and bandwidth, all while being very stable.
High signal power output is desirable because with higher power more attenuation can be tolerated before signal power level falls below a level for satisfactory detection. In addition, the power output distribution should be such that efficient coupling to the fiber is possible given its diameter and numerical aperture. At the other extreme, the power should not be so high as to exceed the material linearity limits unless for some special purpose that is intended.
The power efficiency of the source determines how much input power is required of the pump and, hence, also the heat dissipation requirements. Poor efficiency means higher input power requirements for given optical power output. This can present power supply problems for remotely located equipment. Inefficiently converted source energy also results in excessive heating, requiring appropriate heat dissipation arrangements otherwise unnecessary.
Reliability is of considerable importance, particularly where the optical source is to be used as one of a group in a system. Here both absolute life and mean time between failures (MTBF) are important parameters because they directly influence the reliability of the system overall. Absolute life for most applications is about 100,000 hours while for system applications a satisfactory MTBF is on the order of 10,000 hours or better.
Coherence is a property of sources which gives an indication of the ability of different parts of the wave train emanating from the source to interfere with one another and is of two types, spatial coherence and temporal coherence. The temporal coherence of a wave reflects the narrowness of the frequency spectrum and the degree of regularity of the wave train. Completely coherent light is equivalent to a single-frequency wave train with a frequency spectrum that can be expressed by only a single, monochromatic line. On the other hand, a wave with several frequency components, or a wave that consists of superimposed random short wave trains is said to be incoherent. In practice, it is extremely difficult to achieve a completely coherent wave. Since maximum bandwidth depends on spectral width, it is important for the source temporal coherence to be as good as possible consistent with the objectives of the system.
Cost is a comparative requirement, but it should not be so high that it overly burdens the overall system cost on a comparative basis with competitive systems and should take into consideration not only the cost of the basic device, but any equipment cost associated with the operation of the device itself.
Carrier waves provided by optical sources, like other carrier waves, have information imposed on them through the process of modulation. That is, some optical property of the carrier wave is modified in correspondence with a coding scheme, and the information is subsequently extracted from the carrier wave by suitable encoding techniques. If the output of the source has low temporal coherence, it is difficult to achieve phase and frequency modulation, but intensity modulation in analog and digital form is readily implemented and extensively used.
Modulation can be either internal, i.e., within a light generating source, or external. With an internal modulator, the output of the carrier source, such as a semiconductor laser, is made to vary in accordance with changes in the injection current, which typically serves as the electrical analog of the information signal. External modulators accept a source output as an input and then change some property of the source output for transmission along the fiber trunk line.
The rate of modulation is determined by the speed of the drive circuits and the response time constants of the source or external modulator as the case may be. The faster the response time, the wider the bandwidth signal to be handled.
Those skilled in the art have developed a variety of sources which satisfy the above requirements, some more perfectly than others depending on detailed differences, but all share in common fundamental ideas of operation.
For lasing optical sources, the conditions for laser oscillation in the visible and infrared regions of the spectrum are well understood. Fundamentally, these require that the laser material be capable of fluorescing and that an inversion in population take place between two different energy levels between which the fluorescent emission takes place. There is also the requirement that there be an adequate absorption of the pumping energy to permit pumping action by the light source. In addition, feedback is required through the resonant cavity containing the laserable material.
The ruby laser demonstrated by Maiman in 1960 was single-crystal aluminum oxide "doped" with chromimum impurities. During the intervening years, several crystalline or glass systems with impurity ions as, for example, glass doped with neodymium or other rare earth ions, have been developed.
A large number of gas lasers with outputs in the range from the far IR to the UV are known. Important among these are helium-neon, argon, and krypton as well as several molecular gas systems such as carbon dioxide and molecular nitrogen (N.sub.2).
Solid state semiconductor lasers are known where the electron current flowing across a junction between P- and N-type material produces extra electrons in a conduction band. These radiate upon their making a transition back to the valence band or lower-energy states. If the junction current is large enough, there will be more electrons near the edge of the conduction band than there are at the edge of the valence band and a population inversion may occur.
Aside from the basic known material systems and structures, lasers in the form of optical fibers in which the lasing material has been incorporated into the core have been proposed as, for example, in U.S. Pat. No. 3,958,188 entitled "Fiber Distributed Feedback Laser" issued to James C. Fletcher et al on May 18, 1976 and as disclosed in U.S. Pat. No. 4,044,315 entitled "Means for Producting and Amplifying Optical Energy" issued to Elias Snitzer on Aug. 23, 1977.
In spite of the many innovations made in the laser art, improved laser structures are still required and can be usefully employed in optical fiber communication systems and in other systems, as well, for a variety of applications. Accordingly, it is a primary object of the present invention to provide an improved laser structure.
It is another object of the present invention to provide an improved laser structure capable of being modulated both in frequency and intensity.
It is yet another object of this invention to provide a stable laser having as an output a single narrow line or band of lines where each line in the band is itself narrow.
It is yet another object of the present invention to provide an improved laser structure having narrow output bandwidths at either 0.92, 1.06, 1.34, 1.54, or 2.0 micrometers more or less.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. The invention accordingly comprises the structure exemplified in the detailed disclosure which follows.